JP2583912B2 - Nitrogen oxide removal catalyst - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a catalyst for removing nitrogen oxides, and more particularly to a catalyst for removing nitrogen oxides in which the activity of the catalyst is reduced by heavy metal oxides in exhaust gas.

[Conventional technology]

Nitrogen oxides (NOx) contained in exhaust gas from various combustion furnaces
Is a substance that is not only harmful to the human body itself, but also causes air pollution such as photochemical smog.
In order to remove this NOx (denitration), a catalytic reduction method using NH ₃ (selective reduction) is currently widely used. As a catalyst used in this method, there is a catalyst based on titanium oxide (TiO ₂ ) to which an oxide of a transition metal element such as molybdenum (Mo), tungsten (W), or vanadium (V) is added. . This catalyst has little degradation to sulfur oxides (SOx) and ash contained in exhaust gas,
It is excellent in both activity and life and is currently widely used in practical applications (Japanese Patent Application Laid-Open Nos. 50-51966 and 52-122293).
issue).

[Problems to be solved by the invention]

However, arsenic (As) and selenium (S
If a large amount of vapors of heavy metal oxides such as e), lead (Pb) and tellurium (Te) are contained, the catalyst is poisoned by these vapors, causing a rapid decrease in activity. It is considered that the deterioration of the conventional titanium-based catalyst due to heavy metal oxide vapor is due to the adsorption of these catalyst poison molecules on the active sites on TiO ₂ .

In contrast, the present inventors have proposed a catalyst in which Mo or W oxide is added to TiO ₂ , which is less deteriorated by catalyst poison molecules, but this is because Mo or W oxide covers the TiO 2 surface. This is to protect the active sites on the TiO ₂ from catalyst poison molecules. However, in order to sufficiently bring out the effects of oxides such as Mo or W, firing at a high temperature of 500 ° C or more,
Although it is necessary to sufficiently disperse these oxides on the iO ₂ surface, there is a problem that the specific surface area of the catalyst is reduced by high-temperature calcination. This problem could be overcome by adding silicon oxide (SiO ₂ ) in advance before firing at high temperature and suppressing sintering of TiO ₂ , but on the other hand, the dispersibility of Mo or W oxide was reduced, A new problem arises in that the coating effect is reduced.

An object of the present invention is to solve the above-mentioned problems and to provide a catalyst for removing nitrogen oxides in which the catalytic activity is not reduced for a long time by the vapor of a heavy metal compound contained in exhaust gas.

[Means for solving the problem]

The present invention has been made in order to solve the above-mentioned problems. The first component is titanium oxide, the second component is molybdenum oxide and / or tungsten oxide, and the third component is molybdenum oxide and / or tungsten oxide.
In the nitrogen oxide removing catalyst containing vanadium oxide as a component, the ratio of molybdenum and / or tungsten to titanium is 84.2 to 97.0: 3.0 to 1 in atomic ratio.
5.8, and silicon oxide is 1 to 2 with respect to titanium oxide.
The catalyst is characterized in that it contains 0% by weight, and the mole number of monbdenum oxide and / or tungsten oxide per unit surface area of the catalyst is 5.0 × 10 ^{−6 to} 15.0 × 10 ⁻⁶ mol / m ² .

[Action]

Effect of increasing the SiO ₂ specific surface area of the titanium-based catalyst and, at the same time interfere with the contact between the TiO ₂ particles each other by SiO ₂ particles interrupted between TiO ₂ particles, caused by stabilizing the TiO ₂ particle surface. In general, the more the SiO ₂ particles surround and cover the TiO ₂ particles, the more the specific surface area increases, but the dispersibility of Mo or W oxide on the TiO ₂ surface decreases, and the catalyst for heavy metal oxide vapors Toxicity resistance worsens.

In the catalyst according to the present invention, sintering of TiO ₂ is performed in order that SiO ₂ and TiO ₂ particle surface undergo condensation polymerization to form a three-dimensional network structure and cross-link TiO ₂ particles to prevent contact. Since it is possible to suppress the degree of hiding on the surface of the TiO ₂ particles and to suppress the degree of hiding on the surface of the TiO ₂ particles, it is considered that a decrease in dispersibility of Mo or W oxide on the surface of the TiO ₂ can be minimized.

As the raw material of the second component in the present invention, Mo and W
Oxygen acids, ammonium salts and the like can be used.
In addition, as a raw material of the V oxide as the third component, ammonium metavanadate (NH ₄ VO ₃ ), vanadyl sulfate (VOSO
₄ ) can be used.

In the present invention, the oxide of Mo and / or W is used at a ratio of 84.2 to 97.0: 3.0 to 15.8 in atomic ratio to Ti, and the number of moles of the oxide of Mo and / or W is 5.0 to 5.0 per unit surface area of the catalyst. It is used so as to be in the range of × 10 ^{-6 to} 15.0 × 10 ^-6 mol / m ² .

Yet the SiO ₂ raw material, silicon alkoxide or sol, silicate solution, such as water glass, a slurry, etc. silicate minerals such as quartz can be used, to use alkoxide or sol of silicon of these Is preferred. The addition amount of SiO ₂ is 1 to 20% by weight, preferably 5 to 10% by weight of the amount of TiO ₂ .

As the TiO ₂ raw material used in the present invention, titanic acid, titanium hydrate or titania sol is effective. Such Ti
In a series of processes for producing a catalyst starting from an O ₂ raw material, if TiO ₂ is in a hydrated state, a large number of OH groups present on the TiO ₂ particle surface are converted to alkoxyl groups or OH groups of the SiO ₂ raw material.
It can condense with the group in a three-dimensional network. If it is a state in which TiO ₂ was dehydrated, excessively stabilize TiO ₂ particle surface would be able to SiO ₂ coating film TiO ₂ particle surface, to significantly reduce the dispersibility of the oxide of Mo or W Become.

The catalyst paste obtained by the present invention is formed into various shapes such as plate, honeycomb, granule, cylinder, and column by a wet or dry method, and further applied and formed on a metal or ceramic woven fabric. By doing so, a catalyst molded body can be obtained. Also, to improve the strength,
Inorganic fibers such as ceramics can be added before molding, or various binders can be mixed in the molding process or after molding.

After supporting the oxides of Mo and / or W, and the V oxides or sulfates on the carrier obtained in the above manner,
500-750 ° C. to activate Mo and / or W oxides to increase the catalyst's resistance to heavy metal oxide vapors
Is fired at a high temperature. The optimum range for the calcination temperature of the catalyst depends on the vapor pressure of the added Mo and / or W oxide. For example, 500 to 600 ° C. for Mo oxide, 55
0-750 ° C is preferred. The catalyst obtained by the present invention can maintain a high specific surface area of 60 to ²⁰⁰ m ² / g even after such a high-temperature calcination process.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to specific examples. In the examples,% means% by weight.

Example 1 To 1 kg of a metatitanate slurry containing 30% of TiO ₂ and 2.7% of sulfate groups, 30 g of ethyl silicate (10% of TiO ₂ ) was added and mixed with a kneader for 3 hours. Further, 20.4 g of ammonium metavanadate and ammonium molybdate (3 (N
H ₄₎ by adding _{2 O · 7MoO 3 · 4H 2} O) 77.0g, with heating and kneading to obtain a water content of 32% paste. This was extruded and granulated into a 3 mmφ column, dried at 150 ° C. for 12 hours, pulverized with a hammer mill, and the resulting powder was press-molded to an outer diameter of 13 mm and a thickness of 5 mm with a hydraulic press to 10 to 20 mesh. Crushed. This was put in a crucible and calcined at 550 ° C. for 2 hours in the atmosphere to obtain a granular catalyst.

Examples 2 to 4 In Example 1, the amount of ethyl silicate was changed to 2%, 5%, and 20% of the amount of TiO ₂ to obtain a granular catalyst in the same manner.

Comparative Example 1 A granular catalyst was obtained in the same manner as in Example 1 except that ethyl silicate was not added.

Example 5 In Example 1, the final firing temperature was set to 500 ° C.
Otherwise, a granular catalyst was obtained in the same manner.

Comparative Example 2 A granular catalyst was obtained in the same manner as in Example 1 except that the metatitanate slurry was replaced with powder obtained by previously firing the metatitanate slurry at 500 ° C.

Comparative Examples 3 and 4 A granular catalyst was obtained in the same manner as in Example 1 except that the calcination temperature of the catalyst was 450 ° C. and 620 ° C.

Examples 6 and 7 A granular catalyst was obtained in the same manner as in Example 1, except that the addition amount of ammonium molybdate was 52.1 g and 122.6 g.

Comparative Example 5 A granular catalyst was obtained in the same manner as in Example 1, except that the amount of ammonium molybdate was changed to 0 g.

Example 8 In Example 1, 77.0 g of ammonium molybdate was used.
To ammonium paratungstate ((NH ₄ ) ₆ · W ₇ O ₂₄
・ 6H ₂ O) Changed to 117.6 g, and this was replaced with water 2 containing 46.2 g of oxalic acid
A granular catalyst was obtained in the same manner except that the final catalyst was used at a temperature of 650 ° C.

Example 1 For each of the catalysts obtained in Examples 1 to 8 and Comparative Examples 1 to 5, an adsorption test for determining toxicity was performed under the following experimental conditions. This condition simulates nitrogen oxide removal conditions for coal exhaust gas by introducing diarsenic trioxide, which is generally known to be contained in coal exhaust gas, into the gas.

(1) Diarsenic trioxide adsorption test conditions Gas composition: NO: 200 ppm, NH ₃ : 240 ppm, SO ₂ : 500 ppm, SO ₃ : 50 ppm, As ₂ O
₃ : 140 ppm, CO ₂ : 12%, H ₂ O: 12%, O ₂ : 3%, N ₂ : Balance Adsorption temperature: 350 ° C Adsorption time: 2 hours Space velocity: 184,000h ^-1 (2) Denitration activity calculation method a catalytic amount of measuring conditions and denitrification rate: 10 to 20 mesh particulate catalyst 2cc gas composition was sieved _{to: NO: 200ppm, NH 3:} 240ppm, SO 2: 500ppm, SO 3: 50ppm, CO 2:
12%, H ₂ O: 12%, O ₂ : 3%, N ₂ : Balance Reaction temperature: 350 ° C Space velocity: 184,000h ^-1 The decomposition rate of N (denitration rate) is determined by NO at the inlet and outlet of the catalyst. The change in concentration was measured with a chemiluminescent NOx meter, and calculated by the following equation.

Table 1 shows the denitration rate of each catalyst before and after the adsorption test.

From the results in Table 1, when the catalysts of Examples 1 to 4 and Comparative Example 1 were compared, the specific surface area was increased when SiO ₂ was added in a predetermined range, and the initial activity and the activity after the adsorption test were correspondingly increased. It can be seen that both have improved. In Comparative Example 2,
A TiO ₂ raw material that had been previously calcined at 500 ° C. and dehydrated was used. In this case, a large specific surface area cannot be obtained, and the denitration rate after the adsorption test also has a low value, and the effect of adding SiO ₂ is not seen. In the case of Mo oxide as seen in Comparative Examples 3 and 4 is effective firing temperature of 500 to 600 ° C., this range the effect of SiO ₂ addition is not observed.

From Example 8, it is recognized that W oxide has the same effect as Mo oxide.

FIG. 1 shows Examples 1 to 7 and Comparative Examples 1 to 3 in Table 1.
5 is a plot of the denitration ratio after the adsorption test with respect to the amount of MoO ₃ per unit specific surface area for the catalyst No. 5. It can be seen that all of the catalysts of the examples have a high denitration rate after the adsorption test, and are excellent in toxicity resistance to diarsenic trioxide vapor. In this case, the number of moles of MoO ₃ per unit specific surface area M
Is within the range of 5.0 × 10 ⁻⁶ <M <15.0 × 10 ⁻⁶ mol / m ² .

〔The invention's effect〕

ADVANTAGE OF THE INVENTION According to this invention, when denitrating exhaust gas containing a large amount of vapors of heavy metal oxides such as lead, arsenic, and selenium, a catalyst having high activity and a long life can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the number of moles of MoO ₃ per unit specific surface area of the catalysts shown in Examples 1 to 7 and Comparative Examples 1 to 5 and the denitration rate after the As ₂ O ₃ adsorption test. .

Continued on the front page (72) Inventor Toshiaki Matsuda 3-36 Takara-cho, Kure City, Hiroshima Prefecture Inside the Kure Research Laboratory, Babcock Hitachi Ltd. In the laboratory (56) References JP-A-61-230748 (JP, A) JP-A-62-42744 (JP, A)

Claims (1)

(57) [Claims] 1. A nitrogen oxide removing catalyst containing titanium oxide as a first component, molybdenum oxide and / or tungsten oxide as a second component, and vanadium oxide as a third component. Or the proportion of tungsten is 84.2 to 97 in atomic ratio.
0: 3.0 to 15.8, containing 1 to 20% by weight of silicon oxide with respect to titanium oxide, and having a mole number of molybdenum oxide and / or tungsten oxide per unit surface area of the catalyst of 5.0 × 10 ^-6 to 15.06 × 10 ^-6 mol / m ² , a catalyst for removing nitrogen oxides.